In the manufacturing of, e.g. optical films, it is sometimes necessary to control one or more optical properties quite closely to make acceptable product. For example, the properties of transmission haze, clarity, and transmittance, are frequently of great interest when making films for, e.g. optical panel displays.
Transmission haze is a term used in the art to describe the wide angle scattering of visible light as it goes through a film. Percent haze is generally defined within various standard test methods (e.g., ASTM D1003, ISO 13468) as the ratio of the diffuse light transmittance to the total transmittance through a film or substrate. More specifically, transmission haze is defined as that component of light that is diffusely scattered outside a 2.5 degree angle with respect to a collimated light beam as it transmits orthogonally through the substrate to be measured. Clarity is the optical term used to describe the low angle light scattering that is found within the 2.5 degree angle and transmittance is defined as the ratio of transmitted light through a substrate to the total light incident the film.
Techniques for the measurement of haze and other optical properties are known, and the art describes devices to make such measurements, such as those commercialized by BYK-Gardner GmbH for off-line use. While useful for an analytical lab, these devices are difficult to utilize on-line in manufacturing because of slow response time due to the need to position and remove a cover at the exit port of an integrating sphere for each haze measurement to obtain the total transmitted light through a film as required by ASTM D1003 standard protocol. To make a standalone, absolute measurement of haze, some method to determine total transmitted light through the film is required.
OCS Gmbh (Optical Control Systems) in Germany has developed an on-line sensor based on the same ASTM D1003 standard with the same constraints as the BYK GARDNER device. While suitable for some applications where line speeds are slow, this design only measures haze, and is only configured as a single point measurement. Moreover, the device is big and bulky making it difficult to utilize and incorporate into a scanning frame for on-line uniformity measurement.
The art also describes a portable haze measurement device used to test windscreen material for aircraft. However, this design utilizes a specific wavelength laser source, which may present a problem for materials that scatter more at other wavelengths.
There is an unmet need for an apparatus capable of measuring both haze and clarity over the width of a web moving at commercially viable line speeds.